The present invention relates to a method for regulating arc discharge furnaces which operate with adjustable transformer voltages and adjustable electrode height levels and with an arc voltage regulation which acts directly on the electrode adjustment.
An arc discharge furnace operates with two correcting variables: the transformer voltage or furnace voltage, which can be set at the transformer with the aid of a stepping switch; and the height position of the supporting arm, i.e. the height of the tip of the electrode above the bath (scrap), a factor determinative for the length of the developing arc.
If one disregards asymmetries and their possible compensation by different correcting variables in the three operating power phases, then it is possible to set only two operating parameters independent of one another by means of two regulating elements. For practical furnace operation, the correcting variables of interest could be: the length of the arc, which is approximately proportional to the arc voltage; the arc current; the arc power; and a variable characteristic for the amount of wear of the furnace lining which is calculated from the arc current and the arc voltage. The furnace voltage (transformer voltage), however, can be used only as a correcting variable and not as a regulating parameter since the furnace voltage can serve only as an auxiliary value for the performance of the furnace process.
In practice, the furnace voltage, and thus the transformer setting, is frequently given by the process program. In such cases, the only degree of freedom existing is the setting of the electrode height, i.e. the length of the arc and the arc voltage. For these settings, i.e. the electrode adjustment, the so-called impedance regulation process is employed primarily, i.e. for each electrode there is formed the quotient of the voltage measured between arc current path and the neutral point of the bath divided by the current of the respective arc current path. The quotient value is then the actual value for the impedance regulation. Aside from the fact that this type of regulation has the correct influence on the electrode height level only if the deviations form normal operating states are not too great, this method has the drawback that the impedance is not a reliable measure for arc length.
DE-AS [Federal Republic of Germany Published Application] No. 1,159,112 discloses a power regulation procedure in which the arc length which results in maximum arc power is to be found in that the peak value of the arc voltage to be measured has a certain relationship to the mains voltage. The uncertain manner of compiling the measured values disclosed in this reference already has the result that the object of the invention disclosed in that earlier application is not, or only incompletely, achieved.
Finally, DE-AS No. 2,440,960 discloses wear regulation in an arc furnace in which the actual arc voltage value obtained from a measurement and a value formed of a desired value for the arc resistance and the arc current by means of a multiplier are fed as the desired value to an arc voltage regulator on which is superposed a value which represents the furnace lining wear coefficient as a function of the arc voltage and the arc current, with the output signal of that device forming an additional desired state value for the arc voltage regulator.
Aside from these regulating proposals, some of which have considerable drawbacks, the prior art does not provide an unequivocal teaching of a method for regulating arc discharge furnaces in which the regulating variables are to be used during the corresponding operating states of the arc discharge furnace and with the given values.